System Design And Synchronization Technique Of Spaceborne/Airborne Hybrid Bistatic Synthetic Aperture Radar

In recent years bistatic SAR has been gaining more and more interest because ofsuch specific advantages as reduced vulnerability in military system, anti-disturbingcapability, providing additional information about the target and imaging in flightdirection. Most of the previous studies focused on the systems with symmetricalstructure like spaceborne or airborne bistatic SAR. With the development of thesesystems, bistatic systems with asymmetrical structure are emerging as a new researchtopic.This dissertation carries out research on spaceborne/airborne hybrid bistatic SAR(SA-BSAR), which consists of a radar satellite as the illuminator and a receivermounted on an aircraft. Under the support of the future radar satellite constellation,SA-BSAR may combine high invulnerability and wide coverage provided by thespaceborne transmitter with the great maneuverability and the high resolution providedby the airborne receiver. However, SA-BSAR faces a lot of theoretical andtechnological challenges as well as possessing great potential. First, many correlatedbasis theories must be studied since many of the existing theories, developed forspaceborne or airborne bistatic SAR, are unsuitable for SA-BSAR. Second, the biggestchallenge is the space-time synchronization of the antenna footprints due to the extremeplatform velocity differences. There are two aspects involved: (1) how to improve thescene extension as much as possible, (2) how to compensate the spatial synchronizationerror. Third, similar as other types of bistatic systems, time, frequency and phasesynchronization are also the key techniques to be solved.Focusing on the above challenges, systematical studies have been performed andthe main work and the contributions are presented as follows:1) A relatively comprehensive study on the correlated basis theories are carried out.The research subjects include: the quantitative levels of the main technical parameters,the constraints of the aircraft flying parameters, operation modes, range history, Dopplerparameters, signal model, and etc.2) The influences of the synchronization errors on the imaging performance are analyzed. The necessities of the spatial, time, frequency and phase synchronizationmeasures are confirmed. The requirements on the residual errors are derived.3) Based on an existing spatial synchronization approach relying on 'double slidingspotlight mode' (hereafter referred as DSS approach), a mathematical model for beamsteering is presented and a set of concerned formulae is derived to cancel suchassumptions in DSS approach like fiat-earth geometry, straight-line trajectories, andparallel trajectories between the platforms.4) A new spatial synchronization approach based on 'wide-beam illumination andinverse sliding spotlight receiving mode' is proposed. This approach provides, with thesame azimuth resolution, a longer scene extension than the length achieved by DSSapproach.5) A new spatial synchronization approach based on 'double wide-beam mode' isproposed. This approach avoids a complicated imaging algorithm since it does not needbeam steering. 'wide-beam receiving mode', a special case of'double wide-beam mode',can be applied to a non-cooperative SA-BSAR because satellite beam pattern has not tobe altered.6) An existing approach dedicatedly designed for 'double sliding spotlight mode'and used to compensate the spatial synchronization error, is modified to adapt for theother two operation modes. Grating lobes of the direct-path signal are substituted withmain lobes in the modified approach.7) Several modified and more practical approaches via direct-path signal for timeand frequency synchronization are proposed. The modified approaches directly estimatethe synchronization errors without the assisted means to estimate Doppler centroid ofdirect-path signal. An existing approach used to temporally align the echo data with theorbit data in space-surface bistatic SAR is modified to be suitable for SA-BSAR.Systematic studies are carried out for several key techniques of SA-BSAR,including: 'system theories', 'spatial synchronization', 'time synchronization', and'frequency and phase synchronization'. In the next step, some imaging experiments onthe ground are expected to be performed.